View of The effects of gypsum on the transfer of phosphorus and other nutrients through clay soil monoliths

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The effects of gypsum on the transfer of phosphorus and other nutrients through clay soil monoliths

Risto Uusitalo¹, Kari Ylivainio¹, Jari Hyväluoma¹, Kimmo Rasa¹, Janne Kaseva¹, Pauliina Nylund¹, Liisa Pietola^{2, 3} and Eila Turtola¹

1 MTT Agrifood Research Finland, Plant Production Research, FI-31600 Jokioinen

2 Yara Suomi Oy, Mechelininkatu 1a, P.O.Box 900, FI-00181 Helsinki

3 Present address: MTK (Central Union of Agricultural Producers and Forest Owners), PL 510, FI-00101 Helsinki e-mail: risto.uusitalo@mtt.fi

We applied gypsum (CaSO₄×2 H₂O) amendments to 100 m² plots within two clay-textured fields, one under shallow cultivation to 10 cm depth and the other ploughed to 20 cm depth. Unamended plots and plots subjected to a CaCO₃ (finely ground limestone) application served as controls. Separate soil monoliths (30 cm in diameter, 40 cm in depth) were collected for laboratory rainfall simulations from all plots 7, 19 and 31 months after the initial application of the amendments. Water passed through the monoliths during these simulations was analysed for turbidity, dissolved and particulate phosphorus (DRP and PP), nitrogen species, dissolved organic carbon (DOC), as well as dissolved Ca²⁺, Mg²⁺, K⁺ and S, pH, and electrical conductivity (EC). Over the three-year monitoring period, gypsum amended soils exhibited substantial decreases in turbidity (45%), PP (70%), DRP (50%) and DOC (35%) relative to control samples. The effects gradually decreased with time, and after 31 months gypsum effects on P species were detectible, but no longer statistically significant. We consider gypsum amendments as a potential tool for slowing P loss from agricultural areas with high P loss potential.

Key words: Gypsum, erosion, phosphorus, carbon, nitrogen, leaching, percolation, rainfall simulation, subsurface drainage, agricultural water protection

Introduction

In addition to being a source of Ca and S nutrients for cultivated plants, the mineral gypsum (CaSO₄×2H₂O) has been found to enhance water infiltration in poorly structured soils, inhibit the formation of surface crusts and al- leviate the effects of excessive soil acidity that may restrict root development (see Shainberg et al., 1989). Earlier studies on the use of gypsum in soil and water conservation projects have mostly focused on erosion (e.g., War- rington et al., 1989; Miller, 1987), whereas recent studies have begun to address suppressing phosphorus solubility in high-P soils, and dissolved P (DRP) losses to runoff and percolation water (Zhu and Alva 1994, Stout et al. 2000, Cox et al. 2005). DRP loss abatement and erosion control are especially relevant to non-calcareous and high-P clay soils where unstable clay aggregates can disintegrate and release particles into runoff and percolation water. The dispersed particles represent a relatively large and reactive specific surface area, usually contain more P than the source soil, and can be transported and deposited over large areas, and thus have the potential to contaminate surface waters with agrochemicals and nutrients.

Clay colloids interact according to a set of primary attractive and repulsive forces (see Ryan and Elimelech 1996, Grasso et al. 2002). These interactions are described by the widely used DLVO theory, although other so called

‘non-DLVO’ forces may also affect colloid behaviour (see Grasso et al. 2002). DLVO theory states that clay colloid interactions consist of attractive London–van der Waals forces arising from dipole-dipole interactions, and repulsive electrostatic double-layer forces acting between the negatively charged particles and their surrounding ionic halos. Coagulation or dispersion results from predominance of respective attractive or repulsive forces, as well as from the relative magnitudes of these forces.

Attractive London–van der Waals forces are generally assumed to be independent of changes in the chemical en- vironment but the electrostatic repulsion can be influenced by the ionic strength and composition of a given solution (Ryan and Elimelech 1996, Grasso et al. 2002). Through the influence on the properties of the soil solution gypsum applications can exert primary effects on particle aggregation. Introduction of gypsum to a soil specifically diminishes repulsive forces by compressing the diffuse double layer at the particle surfaces, thus creating a smaller inter-particle repulsion maximum and shorter distance between individual particles. As a result, the relative importance of the attractive forces increases, enhancing colloid aggregation.

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In addition to the effect on colloid aggregation, gypsum may influence the specific adsorption of P onto metal hy- droxide surfaces and edge-positions of silicate minerals by a mechanism similar to that described above for colloid aggregation. In the case of adsorption, compression of the diffuse double layer allows shorter distance between negatively charged mineral surfaces and anions, and thereby increases the rate of P adsorption (see Ryden and Syers 1975, Bar-Yosef et al. 1988). Gypsum as a source of Ca²⁺ may also exert electrolytic effects even if the overall ionic strength of the soil solution remains constant. Comparisons between soils saturated with divalent and monovalent ions (e.g., Ca²⁺ vs. K⁺) show that at constant ionic strength, high Ca²⁺ concentration in the solution increases P retention (Ryden and Syers, 1975). In some circumstances this effect may be partly due to P precipitation as Ca-P complexes (e.g. Zhu and Alva 1994).

Laboratory incubations have shown that gypsum can lower the proportion of water-soluble P in soils with low P retention capacity. Anderson et al. (1995) describe incubation experiments on manure-impacted soils from the Lake Okeechobee area (Florida, USA) in which gypsum caused a 40-63% decrease in P solubility. Short-term incubations of gypsum amended soils showed a similar decrease in P solubility as that reported by O’Connor et al.

(2005). Zhu and Alva (1994) found that gypsum amendments decreased P leaching from loamy sand soil columns by 20–49%. Coale et al. (1994) observed an about 25% decrease in total dissolved P concentration in leachate resulting from a gypsum amendment to an Everglades Histosol (muck). O’Connor et al. (2005) found that a gypsum amendment applied to a packed column of sandy soil from the Lake Okeechobee area apparently lowered P leaching by 33% relative to that observed for the unamended control. These workers further concluded that the relatively high solubility of gypsum made it more effective than water treatment residuals (WWTR’s) containing Al or Fe; the Al/Fe–WWTR’s exhibited only localized effects in retaining and demobilizing P from those parts of the soil column in which they were mixed.

Results from rainfall simulation studies of P retention in gypsum amended soils have varied more widely than those of incubation and column leaching experiments. In their study of grass covered soils from central Pennsyl- vania, Stout et al. (2000) found that gypsum amendments reduced the dissolved P concentrations in runoff by 33%, given 30 minutes of simulated rainfall at an intensity of 50 mm h^-1. Gypsum amendments did not reduce particulate P (PP) detachment from soils having no plant cover and PP was the primary form of phosphorus trans- ferred from the soil into runoff water (Stout et al. 2000). O’Connor et al. (2005) studied the effect of simulated rainstorms (intensity of 71 mm h^-1) on sandy soils from Florida and found that gypsum amendments did not reduce P concentrations in surface runoff from bare or grass sod-covered soil. However, the gypsum application (10 g kg^-1) apparently reduced dissolved P concentrations in water passed through a 7.5 cm soil layer packed into runoff boxes. Favaretto et al. (2006) conducted sequential rainfall simulation experiments (90 min at 30 mm h^-1, followed by 30 min at 60 mm h^-1 intensity) on silty loam soils from the Miami area. These workers found that gypsum amendments increased infiltration of water by 150% and decreased runoff by about 20% relative to the unamended control. Runoff from gypsum amended soils exhibited significantly lower DRP concentrations but overall sediment concentrations were similar to those found in runoff from the control group. Favaretto et al. (2006) also measured nitrogen species in runoff and found that NO₃-N concentrations were similar for both the gypsum amended and unamended control treatments. Concentrations of NH₄-N and particulate N however were lower in runoff from the gypsum amended soils relative to runoff from the control soils.

Rainfall simulation studies referred to above have thus yielded inconsistent results concerning the effects of gypsum on P transport, especially for PP. The high rain intensity used in many rainfall simulations (e.g., Stout et al.

2000, O’Connor et al. 2005, Favaretto et al. 2006) probably strongly influence the results of these studies, as the impact of raindrops on soil surface and rapid dilution of the soil solution abruptly change the physiochemical envi- ronment where P mobilization occurs. Even in cases where the experimental setup accurately simulates processes taking place in areas of high rainfall intensity, translating the results of these studies to regions with low intensity rains such as Finland introduces a significant degree of uncertainty.

This paper describes a series of rainfall simulation experiments designed to investigate the effects of gypsum amendments on percolation water quality on non-calcareous agricultural soils in southern Finland. This study was conducted in tandem with water quality monitoring of a catchment outlet (Ekholm et al. 2012) as well as catchment-scale modelling of effects of gypsum amendments on soil P losses (Jaakkola et al. 2012), and an economic evaluation of the use of gypsum in P-loss abatement (Iho and Laukkanen 2012).

The primary focus of this paper is to evaluate the efficacy of gypsum (CaSO₄×2H₂O) amendments in reducing particle transport as well as nutrient (especially P, but also N and macronutrient cations) and DOC loss from sub-

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drained clay soils that are annually tilled. Ploughing and other cultivation tillage conducted during the fall are the primary means of agricultural soil preparation in Finland. These tilled, fine-grained soils have a high P-loss potential, especially in the form of PP. The fine-grained soils require adequate subsurface drainage to allow field opera- tions (without severe compaction of the soil) during spring and autumn. The majority of soil erosion and nutrient loss from these types of soils occurs through drainflow (see Turtola et al. 2007). Our experiments sought to as- sess the effects of gypsum amendments on drainflow water quality. We assumed that water percolating through undisturbed soil monoliths collected at 40 cm depth was compositionally equivalent to discharge via subsurface drainage pipes. The specific questions addressed by this research are as follows:

1. How much do moderate gypsum applications reduce P transfer through undisturbed clay soils found in the study area?

2. How long do potential P-mitigation effects last and what proxies could be used to monitor the effective life- time of a single gypsum application?

3. What effects do the gypsum amendments exert on the mobilization and leaching of other essential plant nutrients and DOC?

Material and methods

Field sites

The study area consisted of two fields located within a few kilometres of each other at Jokioinen, SW Finland.

These sites are referred to as Fields 1 and 2 and were selected due to their relatively uniform soil texture in about a 100×20 m layout, to allow designation of 12-16 contiguous 5×20 m plots. The soils at both field sites are classified as Vertic Cambisols (IUSS Working Group WRB, 2006) with clayey texture (Table 1).

Table 1. Particle size distribution and carbon content of the Ap horizon (0–25 cm) and the underlying soil layer (25–40 cm) from the two field sites investigated in this study. Results are given as mean values with the range in parentheses.

Particle size distribution (%) C (%)

<0.002 –0.005 –0.02 –0.05 –0.2 –0.5 –2 mm

Field 1 (autumn cultivation to 10 cm depth), n = 10

0–25 cm 49

(41–56) 12

(10–13) 12

(10–14) 16

(11–22) 5

(4–8) 4

(3–5) 3

(2–4) 1.8

(1.5–2.2)

25–40 cm 61

(44–78) 9

(7–12) 9

(4–12) 14

(6–22) 4

(2–7) 1

(1–2) 1

(<1–3) 0.5 (0.4–0.7) Field 2 (autumn ploughing to 20 cm depth), n = 8

0–25 cm 55

(52–57) 18

(17–18) 11

(10–11) 7

(7–8) 3

(3–4) 4

(3–5) 3

(2–4) 3.1

(2.0–4.1)

25–40 cm 66

(63–71) 15

(14–17) 8

(7–9) 6

(6–7) 2

(1–3) 2

(1–2) 1

(<1–2) 1.0 (0.5–1.6)

Agronomic P status of the Ap horizon was classified as “good” for Field 1 (under shallow autumn cultivation) and

“satisfactory” for Field 2 (ploughed). Soil test P concentration (P_Ac) was determined by extraction with acidic ammonium acetate (pH 4.65, Vuorinen and Mäkitie 1955). The P_Ac concentrations and pH for plots subjected to randomized treatments are shown in Table 2, as measured at the beginning of the study and during the final monolith sampling in the spring of 2011.

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Table 2. Soil test P (PAc, ammonium acetate, pH 4.65) concentrations and pH as measured in the Ap horizon of the plots subjected to different treatments. Soil test P and pH are given as mean values, with standard errors in parentheses (n=4). Samples collected in 2008 represent soil conditions before the amendment applications and samples collected in 2011 represent soil conditions at the time of final soil monolith sampling (31 months after the amendments were applied).

Unamended

control Limestone (CaCO₃)

4.3 Mg ha^-1 Gypsum

3 Mg ha^-1 Gypsum

6 Mg ha^-1 Field 1 (cultivation to 10 cm depth)

Aug 2008 P_Ac, mg l^-1

pH 13.6 (1.0)

6.2 (0.05) 12.6 (1.4)

6.3 (0.06) 15.1 (1.7)

6.3 (0.08) 14.0 (1.7) 6.3 (0.07) May 2011

P_Ac, mg l^-1

pH 13.9 (1.9)

6.3 (0.05) 14.7 (2.5)

6.5 (0.004) 14.6 (2.8)

6.1 (0.04) 16.1 (2.9) 6.2 (0.08) Field 2 (ploughed to 20 cm depth)

Aug 2008 P_Ac, mg l^-1

pH 8.4 (0.7)

6.0 (0.02) 9.0 (0.6)

6.0 (0.02) – 8.3 (0.5)

5.9 (0.06) May 2011

P_Ac, mg l^-1

pH 9.0 (1.1)

6.2 (0.09) 10.2 (0.5)

6.2 (0.03) – 8.8 (0.5)

6.0 (0.04)

Gypsum used in this study was obtained from the Yara Suomi Oy plant in Siilinjärvi, eastern central Finland. This gypsum is a side-product of apatite refining/phosphoric acid process and thus contains a small proportion of residual P. The Siilinjärvi ore has a relatively low apatite content of about 10%, and low concentrations of radionu- clides (Karhunen and Vermeulen 2000) and other heavy and/or harmful elements. The concentrations of selected elements as measured in the gypsum used in this study are given below (Aqua Regia-extractable elements):

(g kg^-1) Ca 218, S 180, P 1.6, Fe 0.83, and Na 0.1

(mg kg^-1) K 80, Cu 12, Mg 10, Zn 1.4, Mn 0.5, Pb 3.2, and Cd 0.02

Agricultural lime (ground CaCO₃ rock) was used in this study as an alternative Ca-rich amendment for comparison with gypsum. The origin of this lime was meta-limestones of the Limberg/Skräbböle quarry (Nordkalk Oy Ab, Pargas, SW Finland). Both limestone and gypsum amendments were broadcast in the study plots in October of 2008. Plots were then turned within one week of the application with either a cultivator to a depth of about 10 cm (Field 1), or by ploughing to an approximate depth of 20 cm (Field 2). Tillage procedures were repeated following the autumn harvests in 2009 and 2010.

The soil amendment applications were randomized by splitting the 100×20 m field areas into 4 blocks with one replicate of each treatment randomly applied to each block (thus, four replicate plots at both field sites). Field 1 (cultivation to 10 cm depth) was subjected to four treatments: (i) no amendments (Ctrl), (ii) limestone applied at 4.3 Mg ha^-1 (Lime), (iii) gypsum applied at 3 Mg ha^-1 (Gyp 3), and (iv) gypsum applied at 6 Mg ha^-1 (Gyp 6). The Gyp 6 application (22% Ca-content in gypsum) supplied the same total Ca as the limestone application (30% Ca- content) at 4.3 Mg ha^-1. Field 2 (ploughed to 20 cm) was subjected to treatments i, ii and iv above, but not to the Gyp 3 application (treatment iii).

Fields 1 and 2 were cultivated for spring-sown wheat (Triticum aestivum L. 'Kruunu') during the growing seasons spanned by this study, i.e., crop years 2009 and 2010. The fields were fertilized with mineral fertilizer (saltpetre, NPKS 27-0-1-4) with a N application rate of 100 kg ha^-1. The fertilizer did not include a P application because soil tests indicated a sufficiently high P concentration. The gypsum amendments however supplied around 5 kg P (as residual P) at the lower application level (Gyp 3) and about 10 kg P at the higher rate (Gyp 6) in the autumn of 2008 when the amendments were applied.

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Rainfall simulations

An undisturbed soil monolith measuring 30 cm in diameter and about 40 cm in depth was annually collected from each of the field plots during the month of May starting in 2009 (each spring 16 monoliths from Field 1 and 12 from Field 2, 4 replicates per treatment). The monoliths were collected using a tractor-driven soil auger similar to that described in Persson and Bergström (1991) and PVC cylinders (0.5 m in length) (see online supplementary material, Photographs 1 and 2). The monoliths were capped with soft foam caps, transported to the laboratory and stored in darkness at +6 °C.

Having formed in the post-glacial period with pronounced seasonal variations in water flow and sediment deposi- tion, the soils under investigation have horizontal planes of weakness along slightly variably-grained horizons deposited during spring and autumn. The basal section of most monoliths broke along these stable ped boundaries during sampling. The blade and cutter tip of the coring device however disturbed some sections of the monoliths during sampling. These disturbed basal sections therefore required manual preparation so as to include natural cleavage planes of the soil instead of disturbed soil.

The monoliths were prepared as follows. First, the top of the monolith was stabilized with soft foam, and the soil core was then carefully inverted so that the disturbed parts of the core base could be manually prepared (see online supplementary material, Photograph 3). After preparation, the base was vacuumed, empty spaces filled with washed quartz gravel (about 4–6 mm diameter clasts), and then covered with nylon netting secured in place by an adjustable steel band affixed to the lower section of the core tube. The column was then returned to an up- right position and attached to a 5 cm section of PVC pipe firmly glued to a basal plexiglass plate with a drainage hole and packed with quartz gravel. The overlying soil column was attached to the lower section using a 10 cm wide neoprene strip secured by adjustable steel bands to ensure that the core apparatus was properly sealed.

Each soil core was then slowly saturated from below (via the drainage hole) during one day and kept saturated over additional two days. After this saturation period, the drainage hole of the lower piece of the column apparatus was unstoppered and the soil column drained overnight. The volume of the drained water was measured and sampled. Water samples were frozen and stored at –18 °C for later analysis.

Rainfall was simulated using a stationary drop-former type rain maker as described in Uusitalo and Aura (2005).

This device pumps deionized water through a column maintained at a constant water level into 0.51 mm diameter capillary tubes. The capillary tubes (96 in total) attach to a 1×1 m steel frame to produce droplets weighing around 37.5 mg at a desired interval (see online supplementary material, Photographs 4 through 6). The fall height in our simulations was set at 2.4 m. Based on drop size and fall height, we estimated the kinetic energy of the simulated rain to be about 80 J m^-2 h^-1. The fall height was not sufficient for the drops to reach terminal velocity but yielded kinetic energy similar to that observed in natural rainfall of the same (5 mm h^-1) intensity (cf. Salles et al. 2002); the larger droplet size of the simulated raindrops compensates for their lower impact velocity relative to natural raindrops. Simulated rainfall was applied for 5 hours per day over two consecutive days at an intensity of 5 mm h^-1 (25 mm of rain per day and 50 mm total). A 5 mm h^-1 rainfall intensity is typical of conditions in SW Finland (see Kuusisto 1980). Climatological and meteorological data from the Jokioinen observatory show that daily precipitation exceeded 20 mm for an average of 2.4 days per year from 1991–2001 (see Turtola et al. 2007).

Higher daily precipitation is less common: 50 mm of rain in one day for example would only occur once every 10 years (Venäläinen et al. 2009).

Four water samples from each soil core were collected during the rainfall simulations: (0) overnight drainage water (i.e., water that was collected from the initial saturation of the soil cores), (1) percolation water collected during the first day of simulated rain, (2) percolation water collected from overnight drainage following the first day of simulated rain, and (3) percolation water collected during the second day of simulated rain. The numbers in parentheses above designate each water sample along the x-axes of Figures 1 through 3. The water samples were subdivided into two fractions. The first fraction was immediately filtered through 0.2 µm Nuclepore (What- man, Maidstone, UK) membranes. Both filtered and unfiltered subsamples were stored at –18 °C for later analysis.

The filtered subsamples were analysed for DRP, NO₃-N, NH₄-N, Ca²⁺, K⁺, Mg²⁺, and S. Sulphur concentrations and knowledge of the initial filtering technique used suggest that S consisted mostly of SO₄-S, with a minor contribu- tion from organosulphur species within the dissolved organic matter fraction. The unfiltered subsamples were analysed for total P and N concentrations following autoclave induced digestion with peroxodisulphate and sulphuric acid. The differences between the concentrations of total and dissolved forms were taken to represent particu-

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late P (PP) and organic N (N_org). Phosphorus and nitrogen species were analysed with a LaChat analyser (Milwau- kee, WI, USA) while Ca²⁺, Mg²⁺, K⁺ and S were measured using an ICP-AES (Thermo Jarrel Ash, Franklin, MA, USA).

For dissolved organic carbon (DOC) analyses, samples were passed through Whatman GF/C glass filters (instead of Nuclepore membranes) and then analysed with a Shimadzu TOC analyser (Tokyo, Japan). Turbidity of percolation water was recorded immediately following collection using a Hach 2100 AN IS Turbidimeter (Loveland, CO, USA).

This instrument measures nephelometric turbidity units (NTU) over an undiluted range of up to 10⁴ NTU. For a set of about 50 water samples, total suspended solids (TSS) were additionally determined by weighing evapora- tion residue. Complementary turbidity and TSS data allowed us to calculate TSS as a function of turbidity measurements (NTU):

TSS (g l^-1) = –4.414×exp(–0.0001617×NTU) + 4.558 (r² = 0.96) (Eq. 1) Electrical conductivity (EC) was measured with an Orion 150 conductivity meter (cell 012210), and pH was recorded with an Orion 420A meter (pH electrode 9457BN; Thermo Electron Corp., Waltham, MA, USA).

Statistical analyses

Statistical analyses were performed using the SAS software package (SAS Institute, Inc., Cary, NC, USA). Variables with normally distributed residuals such as pH, or those that could be normalized by logarithmic (DRP, DOC) or square root (PP, TP, N forms, K) transformations were analysed using the SAS ‘MIXED’ procedure which executes a Restricted Estimation of Maximum Likelihood (REML) method. Variables for year, treatment and a within-simulation samples variable (abbreviated below as “WSS”) were denoted as fixed effects. The WSS variable, compound- ing individual water samples (0, 1, 2, 3) taken at different times during the simulations, was fixed because it exhibited trends during the course of the simulation; the trends in concentrations are shown in Figures 1 through 3.

The model takes into account that year and WSS were repeated measures having compound symmetry (CS) and unstructured (UN) covariance structure, respectively. The model can thus be expressed in equation form as follows:

y_ijkl= μ + B_{l +}T_i+ TB_il+ Y_j+ YB_jl+ TY_ij+ S_k+ SB_kl+ TS_ik + TSB_ikl+ TYS_ijk+ TYSB_ijkl+ YS_jk + YSB_jkl+ εijk (Eq. 2) where μ is the overall mean, and T_i, Y_j and S_kare the fixed effects of the treatment, year and WSS, respectively.

Two- and three-factor interactions of fixed effects were also included in the model. The terms SB_kl, TB_il, YB_jl, TSB_ikl, YSB_jklandTYSB_ijkl represent random effects of interaction of the B_l (block) term with other factors, and ε_ijkl is the residual error. The Tukey-Kramer post-hoc test was used to compare means.

Certain variables (turbidity, EC, Ca²⁺, Mg²⁺, S) had demonstrably non-normal residual distributions and could not be normalized by log or square root transformations. These variables were analysed with the non-parametric Kruskall-Wallis test and by post-hoc comparison of contrasts between the treatments. Data from each field was analysed separately to account for the different depths of autumn tillage.

Results

Turbidity of percolation water

Turbidity levels and the treatment effects on turbidity were clearly variable over the three-year study period (Fig.

1, Table 3). The effect of the gypsum amendment on the turbidity of percolation water was especially dramatic for the first rainfall simulation (monolith sampling at 7 months after gypsum and limestone applications; online supplementary material, Photograph 7). Both levels of gypsum application (Gyp 3 and Gyp 6) were associated with dramatic decreases in turbidity, whereas the limestone application (Lime) had no apparent effect on turbidity. In the second year’s simulation (at 19 months after amendment applications) turbidity was relatively high for water percolated through the monoliths (Fig. 1, Table 3); this effect was probably due to higher soil moisture levels during the second monolith sampling. At that time water samples from the gypsum-amended soil monoliths also exhibited only about 50% of the turbidity observed in those of the unamended control (Ctrl) or Lime soil. Turbidity generally increased with time as the second year’s simulation proceeded (Fig. 1). By 31 months, water collected from initial saturation of gypsum amended soils (the 0 samples in Fig.1) had slightly lower turbidity than that from other treatments, but average values of the third year’s simulations were statistically insignificant due to variation measured from replicate soil cores.

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0 1 2 3 0 1 2 3 0 1 2 3 0.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35

DRP, mg l-1

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm) 0 1 2 3 0 1 2 3 0 1 2 3 0

500 1000 1500 2000 2500 3000 3500 Ctrl

Lime Gyp 6Gyp 3

NTU

0 1 2 3 0 1 2 3 0 1 2 3 0.0

0.5 1.0 1.5 2.0

PP, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3 0.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35

DRP, mg l-1

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm) 0 1 2 3 0 1 2 3 0 1 2 3 0

500 1000 1500 2000 2500 3000 3500

NTU

0 1 2 3 0 1 2 3 0 1 2 3 0.0

0.5 1.0 1.5 2.0

PP, mg l-1

Field 1, shallow cultivation Field 2, ploughed

Overall the results show that gypsum applications significantly reduced the turbidity of percolation water samples. The study-averaged turbidity of water samples from gypsum amended soil was 36-50% lower than that of water samples from the Ctrl and Lime soils (Table 3). For the two gypsum amendment levels applied to Field 1, the Gyp 3 application resulted in a mean 43% reduction, and Gyp 6 resulted in a mean 49% reduction in turbidity. For Field 2 (Gyp 6) the respective turbidity reduction was 40%. The unequal response of Field 1 and 2 to the Gyp 6 application is likely due to the different mixing depths of gypsum for ploughing versus cultivator tillage.

Estimates for treatment-averaged TSS (total suspended solids) concentrations were calculated using Eq. 1 and turbidity data (Table 3). Results of this procedure indicate that water samples from the Ctrl and Lime amended soils had TSS concentrations of about 0.8-1.2 g l^-1. Water samples from the gypsum amended soils gave estimated TSS concentrations of 0.5-0.7 g l^-1. The lowest turbidity measurement belonged to a water sample from the first sampling (7 months) of a gypsum amended soil and corresponded to a TSS concentration of about 0.2 g l^-1.

Fig. 1. Turbidity (NTU), particulate P (PP) and dissolved reactive P (DRP) concentrations in percolation water collected from soil monoliths (Fields 1 and 2) subjected to two-day rainfall simulations. Soil monoliths were collected 7, 19 and 31 months after limestone and gypsum applications. The markers show mean measurements for four replicate soil monoliths collected from plots of the unamended control soil (Ctrl), plots subjected to the limestone (Lime), the 3 Mg ha^-1 gypsum (Gyp 3), and the 6 Mg ha^-1 gypsum (Gyp 6) applications,. The subsets of x-axis markers indicate the four different water samples taken throughout the experiment: 0 – water draining overnight following the initial saturation of the soil cores; 1 - percolation water obtained during the first rainfall simulation event; 2 - water draining overnight following the first rainfall simulation event; and 3 - percolation water obtained during the second rainfall simulation event. On both days, simulations lasted for 5 hours with 5 mm h^-1 rainfall intensity. The numbers in parentheses beneath the x-axis labels give the cumulative natural rainfall (mm) in the field area following the initial amendment application.

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Table 3. Turbidity, and concentrations of particulate P (PP) and dissolved reactive P (DRP) in percolation water samples obtained from 40-cm deep undisturbed soil monoliths subjected to rainfall simulations. The monoliths were sampled from Fields 1 and 2 over three consecutive spring periods (7, 9 and 31 months) following an initial limestone and gypsum application. The numbers given in the table represent averages from four different water samples collected during two-day rainfall simulations. The mean concentrations of water samples collected from soils subjected to each treatment and averaged over the entire study are shown at the bottom. The superscript tags indicate significant (p<0.05) differences between the treatments; mean values that do not differ statistically significantly are marked with the same letters. The different treatments include the unamended control soil (Ctrl), limestone amended soil (Lime), the 3 Mg ha^-1gypsum application (Gyp 3), and the 6 Mg ha^-1gypsum application (Gyp 6). N.A. = not applicable (the 3 Mg ha^-1gypsum amendment was not applied to Field 2 plots).

Field 1 (cultivation to 10 cm depth) Field 2 (ploughed to 20 cm depth)

Turbidity PP DRP Turbidity PP DRP

NTU –––––––───mg l^-1 –––––––─ NTU –––––––─ mg l^-1 –––––––──

7 months after amendment applications (n = 16)

Ctrl 1148â 1.157â 0.223â 716â 0.792â 0.126

Lime 1217â 1.034â 0.189â 388â,b 0.502â,b 0.137

Gyp 3 145^b 0.093^b 0.021^b N.A. N.A. N.A.

Gyp 6 39^b 0.098^b 0.054^b 109^b 0.182^b 0.074

Ctrl 2153â 1.521â,b 0.117â 1717â 0.846 0.119

Lime 2352â 1.540â 0.209â,b 1609â 0.781 0.073

Gyp 3 1486â,b 0.933â,b 0.120â,b N.A. N.A. N.A.

Gyp 6 1220^b 0.480^b 0.051^b 1221^b 0.390 0.034

Ctrl 1296 1.110 0.154 752 0.660 0.037

Lime 1315 0.657 0.159 754 0.579 0.080

Gyp 3 1079 0.838 0.131 N.A. N.A. N.A.

Gyp 6 1159 0.523 0.073 433 0.399 0.043

Mean values averaged over the entire study (n = 48)

Ctrl 1532â 1.256â 0.159â 1052â 0.764â 0.084â

Lime 1628â 1.046â 0.185â 917â 0.615â 0.093â

Gyp 3 903^b 0.531^b 0.073^b N.A. N.A. N.A.

Gyp 6 806^b 0.332^b 0.059^b 588^b 0.314^b 0.048^b

Phosphorus concentrations in percolation water

In a manner similar to that of turbidity, trends in P concentrations in water samples from soils collected throughout the study differed over time. Water samples of the first year simulations (7 months) were highly different in their PP concentrations, with the lowest values in gypsum amended soils (Fig. 1, Table 3). At 19 months (second spring), water samples from Field 1 plots subjected to the Gyp 6 application still exhibited lower PP concentrations than those representing Ctrl and Lime treated soils. Water from Field 2 plots subjected to the Gyp 6 application however did not show statistically significant differences in PP concentrations relative to values associated with the other treatments (Table 3). By 31 months (third spring), the PP concentrations associated with the gypsum amendment were still on average about half of those of the Ctrl or Lime amended soils, but these differences were not statistically significant given the standard errors of the mean values.

Taken over the whole duration of the study, PP concentrations in water collected from the gypsum amended soils were 49─74% lower than those of Ctrl and Lime soils (Table 3). The gypsum applications to Field 1 were followed by PP concentrations that were 53% (Gyp 3) and 71% (Gyp 6) lower on average (respectively) than values associated with other treatments. For Field 2, PP concentrations from Gyp 6 were 54% lower than for the Ctrl.

The response of the treatments in DRP concentrations also varied through time of monolith sampling. When the

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tion water than those of the Ctrl and Lime. The post-hoc test however only identified significant treatment effects within a given sampling among Field 1 percolation water samples at 7 and 19 months samplings. On the contrary, the statistical test failed to identify any significant yearly differences between the treatments for Field 2 (Table 3).

When averaged over the 31-month duration of the study, water samples from gypsum amended soils had DRP concentrations that were 43─68% below those associated with Ctrl and Lime amended soils (Table 3). Water samples from Lime soils had slightly (10-14%) higher average DRP concentrations relative to those of the Ctrl. Water samples from Field 1 soils subjected to the Gyp 3 and Gyp 6 applications had DRP concentrations that were 57%

and 66% lower,respectively, than that of water from the Ctrl. For Field 2 the respective reduction in the DRP concentrations for Gyp 6 was 46%.

Mobility of DOC and N species

The concentrations of DOC in water samples from gypsum amended soils were consistently lower than those of the Ctrl and Lime soils for each year of the study (Fig. 2). These differences were statistically significant at 7 and 19 months but became non-significant at 31 months. Water samples from the Lime soil yielded the highest DOC concentrations among the treatments (Table 4). When averaged over the duration of the study, DOC values in water samples from gypsum amended soils were 32─45% lower than those associated with the Ctrl and Lime soils.

0 1 2 3 0 1 2 3 0 1 2 3

0.00.5 1.01.5 2.02.5 3.03.5 4.0

Norg, mg l-1

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm)

0 1 2 3 0 1 2 3 0 1 2 3

0 5 10 15 20 25

30 Ctrl

Lime Gyp 3 Gyp 6

DOC, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3

0.00.5 1.01.5 2.02.5 3.03.5 4.0

Norg, mg l-1

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm)

0 1 2 3 0 1 2 3 0 1 2 3

0 5 10 15 20 25 30

DOC, mg l-1

The concentrations of organic N (N_org, calculated as the difference between total N and the sum of NO₃-N and NH₄-N) in water samples from gypsum amended soils were lower than those measured in water samples from the Ctrl and Lime soils (Table 4). Statistically lower concentrations of N_org were found for Field 1 for both Gyp 3 and Gyp 6 at 7 months and for Gyp 6 at 19 months (Table 4). Water samples collected from Field 2 monoliths gave N_org concentrations that did not differ significantly from each other, given a 95% confidence interval. The post-hoc test comparison of N_org concentrations for the Gyp 6 amendment and that of Ctrl collected at 7 months however gave a p-value of 0.053. When averaged over the entire study, N_org concentrations in water samples from gypsum amended soils were 35─47% lower than corresponding values associated with Ctrls. Both N_org and DOC concentrations followed trends similar to each other (Fig. 2).

The NO₃-N and NH₄-N concentrations in percolation water did not show statistically significant differences. Rela- tive to other treatments the averaged concentrations of NO₃-N were generally higher for gypsum amendment in Field 1, but lower in Field 2. The pattern in NO₃-N concentrations for Field 1 versus Field 2 samples appeared in

Fig. 2. The concentrations of dissolved organic C (DOC) and of organic N in percolation water samples collected from soil monoliths (Fields 1 and 2) subjected to two-day rainfall simulations. For the other labels, see the caption of Fig. 1.

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Table 4. The concentrations of dissolved organic C (DOC) and organic N (Norg) in percolation water obtained from soil monoliths subjected to rainfall simulations.

For the other information, see the heading of Table 3.

Field 1 (shallow cultiv.) Field 2 (ploughed)

DOC Norg DOC Norg

––––––– mg l^-1 ––––––– ––––––– mg l^-1 –––––––

Ctrl 18.7â 2.25â 22.5â 2.99

Lime 19.8â 2.17â 26.4â 2.87

Gyp 3 8.2^b 0.75^b N.A. N.A.

Gyp 6 9.4^b 0.86^b 14.18^b 1.74

Ctrl 13.7â,b 3.01â,b 21.82â 3.31

Lime 18.1â 3.44â 19.16â,b 3.06

Gyp 3 10.7^b 2.28^a,b N.A. N.A.

Gyp 6 9.2^b 1.86^b 12.70^b 2.16

Ctrl 17.0 2.07 16.38 2.08

Lime 17.8 1.78 20.40 2.09

Gyp 3 14.5 1.80 N.A. N.A.

Gyp 6 11.8 1.24 14.08 1.52

Mean values averaged over entire study ( n = 48)

Ctrl 16.3â 2.43â 20.1â 2.77â

Lime 18.5â 2.42â 21.9â 2.66â

Gyp 3 10.9^b 1.53^b N.A. N.A.

Gyp 6 10.1^b 1.29^b 13.6^b 1.74^b

Table 5. The concentrations of dissolved NO₃-N and NH₄-N in percolation water obtained from soil monoliths subjected to rainfall simulations. For the other information, see the heading of Table 3.

NO₃-N NH₄-N NO₃-N NH₄-N

––––––– mg l^-1 ––––––– ––––––– mg l^-1 –––––––

Ctrl 1.386 0.029 6.581 0.057

Lime 1.232 0.028 5.404 0.082

Gyp 3 2.576 0.043 N.A. N.A.

Gyp 6 2.171 0.019 4.985 0.054

Ctrl 1.545 0.026 2.331 0.058

Lime 1.341 0.033 2.674 0.026

Gyp 3 1.185 0.038 N.A. N.A.

Gyp 6 2.086 0.046 2.036 0.026

Ctrl 1.687 0.065 3.098 0.037

Lime 2.072 0.046 1.834 0.084

Gyp 3 1.908 0.052 N.A. N.A.

Gyp 6 2.716 0.049 2.011 0.060

Mean averaged over the entire study; n = 48)

Ctrl 1.537 0.038 3.805 0.050

Lime 1.527 0.035 3.138 0.058

Gyp 3 1.845 0.044 N.A. N.A.

Gyp 6 2.316 0.036 2.865 0.044

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Electrical conductivity, other nutrients, and pH

At 7 months the EC values in percolation water were around 155─190 µS cm^-1 from the Ctrl and Lime treatments.

In Field 1 they were twice as high (350 µS cm^-1) for the Gyp 3, and four times as high (745─820 µS cm^-1) for the Gyp 6 application (Fig. 3, Table 6). At 19 months EC decreased markedly relative to the previous spring, and con- tinued to decrease moderately thereafter. Even at 31 months the EC values associated with the Gyp 6 application were greater than those of the Ctrl.

Trends observed in EC values were similar to those observed in Ca²⁺ and S concentration data (Fig. 3). Unlike P, N and DOC concentrations, EC (along with Ca²⁺ and S) of the Gyp 6 application exhibited significant differences relative to the Ctrl throughout the 31-month study period (Table 6).

0 1 2 3 0 1 2 3 0 1 2 3 0

25 50 75

100 Ctrl Gyp 3

Lime Gyp 6

Ca, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3 0

25 50 75 100 125

S, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3 0

25 50 75 100 125 150

Ca, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3 0

25 50 75 100 125 150

S, mg l-1

0 1 2 3 0 1 2 3 0 1 2 3 0

200 400 600 800 1000

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm)

EC,µS cm-1

0 1 2 3 0 1 2 3 0 1 2 3 0

200 400 600 800 1000

7 mo(340 mm) 19 mo

(835 mm) 31 mo (1380 mm) EC,µS cm-1

Fig. 3. The concentrations of dissolved Ca²⁺ and total dissolved S, and electrical conductivity (EC) of percolation water collected from soil monoliths (Fields 1 and 2) subjected to two-day rainfall simulations. For the other labels, see the caption of Fig. 1.

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Table 6. Electrical conductivity (EC) and the concentrations of dissolved Ca and S in percolation water obtained from soil monoliths subjected to rainfall simulations. For the other information, see the heading of Table 3.

Field 1 (cultivation to 10 cm depth) Field 2 (ploughed to 20 cm depth)

EC Ca²⁺ S EC Ca²⁺ S

µS cm^-1 ––––––– mg l^-1 ––––––– µS cm^-1 ––––––– mg l^-1 –––––––

Ctrl 151^b 16.5^b 5.9^b 190^b 20.0^b 5.5^b

Lime 149^b 15.5^b 4.7^b 273^a 34.0^a 6.3^b

Gyp 3 348â 33.1â 42.9â N.A. N.A. N.A.

Gyp 6 744â 93.2â 117.3â 816â 121.5â 126.7â

Ctrl 143^b 13.7^c 6.5^b 161^b 16.8^b 6.3^b

Lime 162^b 17.5^b,c 5.4^b 189^b 20.3^b 7.8^b

Gyp 3 235â 23.9â,b 24.7â N.A. N.A. N.A.

Gyp 6 369â 34.2â 40.5â 356â 40.7â 42.3â

Ctrl 154^b 16.3^b 7.5^b,c 167^b 17.5^b 7.7^b

Lime 161^b 18.0^b 5.8^c 199^a,b 23.0^a,b 6.8^b

Gyp 3 197^b,a 20.8^a,b 16.0^a,b N.A. N.A. N.A.

Gyp 6 320â 31.1â 31.9â 301â 31.9â 33.0â

Mean averaged over entire the study ( n = 48)

Ctrl 149^c 15.5^c 6.6^c 173^c 18.0^c 6.5^b

Lime 157^c 17.0^c 5.3^d 220^b 25.8^b 6.9^b

Gyp 3 260^b 25.9^b 27.8^b N.A. N.A. N.A.

Gyp 6 478â 52.9â 63.2â 491â 64.7â 67.4â

Averaged Mg²⁺ concentrations associated with the Ctrl ranged from 6.7-9.4 mg l^-1 while those of the Gyp 3 and Gyp 6 amended monoliths were twice and three times as high, respectively (Table 7). Comparison of annual variation in Mg²⁺ data shows that concentration peaked in the first year and then declined. Mg²⁺ concentrations re- mained significantly higher in water samples from the Gyp 6 application (relative to the values of the Ctrl) even after 31 months.

Concentrations of K⁺ were apparently higher in water samples from the Gyp 6 applications but the overall treatment effect was statistically significant only for samples from Field 1 plots. The K⁺ concentrations in water samples from soils receiving the Gyp 6 application were specifically 30─50% higher than values associated with all other treatments, including the Gyp 3 application. Comparison of annual variation in K⁺ data indicates that the only significant difference in concentrations exist between water samples from the Gyp 6 application and those of the Ctrl (Table 7). For Field 2, data on K⁺ concentrations showed no significant annual differences among water samples representing the different soil treatments.

Soil pH (1:2.5 vol/vol soil–water suspension) as measured in the Ap horizon (6.0─6.5) showed relatively small variations between the treatments, or during the course of this study (Table 2). Percolation water from the soils analysed here ranged in pH from 7.0 to 8.0 (Table 8), thus clearly higher values than in soil suspensions. The lowest values, observed in water samples from the first year (7 months) soil monoliths subjected to the Gyp 6 application, were significantly lower than other pH values. The highest pH values were associated with Lime soils (Table 8). Water samples from gypsum amended soil collected during the second and third years had pH values similar to those of the Ctrl.

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Table 8. Potential of hydrogen (pH) in percolation water obtained from soil monoliths subjected to rainfall simulations. For the other information, see the heading of Table 3.

Field 1 Field 2

pH pH

7 months after application

Ctrl 7.5^a,b 7.4^b

Lime 7.6^a,b 8.0^a

Gyp 3 7.7^a N.A.

Gyp 6 7.3^b 7.0^b

Ctrl 7.7 7.8

Lime 7.9 8.0

Gyp 3 7.6 N.A.

Gyp 6 7.7 7.7

Ctrl 7.7 7.7

Lime 7.8 7.9

Gyp 3 7.5 N.A.

Gyp 6 7.6 7.5

Mean values for entire study

Ctrl 7.6^a,b 7.6^b

Lime 7.8^a 8.0^a

Gyp 3 7.6^a,b N.A.

Gyp 6 7.5^b 7.4^c

monoliths subjected to rainfall simulations. For the other information, see the heading of Table 3.

Mg²⁺ K⁺ Mg²⁺ K⁺

––––––– mg l^-1––––––– ––––––– mg l^-1–––––––

Ctrl 6.6^b 3.50^a,b 8.0^c 3.41

Lime 6.4^b 3.61^a,b 10.7^b 4.35

Gyp 3 18.8^a 2.25^b N.A. N.A.

Gyp 6 38.8â 7.41â 33.8â 6.56

Ctrl 6.1^b 3.60 6.3^b 2.23

Lime 6.1^b 5.18 8.2^b 1.76

Gyp 3 10.3^a 4.83 N.A. N.A.

Gyp 6 15.0^a 4.70 15.9^a 2.41

Ctrl 7.4^b 2.78 8.3^b 1.41

Lime 7.5^b 2.71 9.4^a,b 1.47

Gyp 3 8.9^a,b 3.36 N.A. N.A.

Gyp 6 15.1^a 3.35 14.0^a 1.94

Mean averaged over the entire study (n = 48)

Ctrl 6.7^c 3.28^b 7.5^c 2.21

Lime 6.7^c 3.76^a,b 9.4^b 2.24

Gyp 3 12.7^b 3.40^a,b N.A. N.A.

Gyp 6 23.0â 5.02â 12.1â 3.12